All wheel drive systems have been known for years in the motor vehicle industry. A typical type of all wheel drive vehicle is a full time auto engaging system wherein a fixed torque split of e.g., 50/50 between the front and rear axles is used. However, other torque splits such as 60/40, 70/30 or even 20/80 may be used. Most current all wheel drive systems have a center differential which provides the torque to the rear differential and front differentials, respectively. The center differential on prior art all wheel drive vehicles can be combined with a low range gear for use in off road driving and severe driving conditions. The low range gear in the transfer case of prior art devices provides extremely high torque to the wheels for heavy duty towing and hazardous driving conditions. Some of the all wheel drive prior art vehicles have also provided the low range gear as a shiftable planetary reduction gear that is integrated in the transfer case of the drive line system. The use of the planetary gear arrangement in the center differential generally requires an extended transfer case which creates a need for all wheel drive vehicles to have longer power trains that lead to increases of the noise, vibration and harshness of the vehicle along with packaging issues associated with such low range options.
The prior art all wheel drive systems that include low range gears also have to design all of the drive line components after the reduction gear to be sized for maximum load which leads to a high torque, high weight drive line, thus the need for specifically designed prop shafts, axles and side shafts for the drive line system. The prior art all wheel drive vehicles also have drive trains that may have transversal engine arrangements which restricts the amount of packaging for the transfer case which in some instances does not even allow for the integration of the low range option because of packaging restrictions with the engine arranged in such a manner. Furthermore, these prior art all wheel drive systems use the torque split between the front and rear axles for both on and off road usage, i.e., whether in the high gear or the low range gear, which is not conducive to optimum all wheel drive conditions in the low range option. The all wheel drive systems also tend to need forced lubrication for the low range permanent idling reduction gears which usually includes an oil pump which will increase the cost, complexity and losses in the transfer case of the all wheel drive system. The prior art all wheel drive systems can either be completely mechanical wherein the secondary driven axle is a hang on axle that is engaged when the primary driven axles slips and such slip is sent through a viscous coupling or other differential setup that engages the rear differential thus sending torque to the wheels that have the better grip in the road conditions. However, with the complexity of all wheel drive vehicles increasing there are also systems that electronically sense the slip on the individual wheels of the front axle and the rear axle and electronically engage the brakes on the slipping wheels to send power to the other wheels within the predetermined torque split. Many of these brake and traction controlled full time all wheel drive systems are left without the option of a low range because of the packaging issues and transversal engine arrangement found in a lot of the smaller SUV's.
Therefore, there is a need in the art for an all wheel drive system that includes a center differential and has a low range gear integrated in one of the drive axles. This will allow for a full time all wheel drive system to include a low range option in all vehicle configurations including transversal engines, etc.